1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to link adaptation for multi-user wireless communication.
2. Introduction
In some types of multiple access wireless communication systems, an access point (e.g., a base station) provides network connectivity and other services for access terminals (e.g., cell phones, computers, etc.) in the vicinity of the access point. In some cases, the access point may communicate using a single-user (SU) mode (e.g., using beamforming to communicate with a given access terminal) or using a multi-user (MU) mode (e.g., using multi-user multiple input multiple output (MU-MIMO) to concurrently communicate with several access terminals).
A MU-MIMO mode of operation may be used to enable concurrent communication between an access point and multiple access terminals. For example, in an IEEE 802.11ac compliant system, an 802.11ac base station may employ MU-MIMO to communicate with several stations. An access point of a MIMO system employs multiple antennas for data transmission and reception while each access terminal employs one or more antennas. The access point communicates with the access terminals via forward link channels and reverse link channels. A forward link (or downlink) channel refers to a communication channel from a transmit antenna of the access point to a receive antenna of an access terminal, and a reverse link (or uplink) channel refers to a communication channel from a transmit antenna of an access terminal to a receive antenna of the access point.
MIMO channels corresponding to transmissions from a set of transmit antennas to a receive antenna are referred to as spatial streams since precoding (e.g., beamforming) is employed to direct the transmissions toward the receive antenna. Consequently, in some aspects each spatial stream corresponds to at least one dimension. A MIMO system provides improved performance (e.g., higher throughput and/or greater reliability) through the use of the additional dimensionalities provided by these spatial streams.
The quality of the channel between the access point and each of the access terminals is generally taken into account when selecting transmission parameters (e.g., modulation and coding scheme (MCS)) for transmissions from the access point to the access terminals. For example, the access point may send a training sequence to the access terminals, and request each access terminal to provide feedback including a channel estimate derived from the training sequence and a transmission parameter estimate that is based on that channel estimate. The access point may then use these transmission parameter estimates to control subsequent transmissions to the access terminals. As a specific example, an 802.11 base station broadcasts a null data packet announcement (NDPA) frame including an MCS request (MRQ), followed by the null data packet (NDP). A station responds to this request with MCS feedback (MFB). To support such a link adaptation scheme, various specifications have been defined for managing the transmission parameters.
For example, under 802.11, if MFB is sent in the same PLCP Protocol Data Unit (PPDU) as a Very High Throughput Compressed Beamforming (VHT-CB) frame of type SU, the MFB responder shall estimate the recommended MFB under the assumption that the MFB requester will use the steering matrices (e.g., channel estimate) indicated by the VHT-CB frame. Also, if the MFB requester sends the MRQ in an NDPA requesting SU-beamforming feedback, then the MFB responder shall include the corresponding MFB feedback in the response VHT-CB frame.
While the above specifications address SU mode, these specifications do not address link adaptation for MU mode. Accordingly, there is a need for effective techniques for providing link adaptation for MU mode.